Recently, a communication system using the W-CDMA (Wideband Code Division Multiple Access) scheme is developed as third-generation mobile communication means, and services such as voice communication and packet communication are provided from an OSI (Open Systems Interconnection) protocol based mobile communication network to mobile communication terminals such as a mobile phone. On a mobile communication terminal such as a mobile phone corresponding to the W-CDMA scheme, the SIR(Signal to Interference Ratio) is used as a parameter for deciding the state of synchronization between the local spread code allocated to a local terminal and the reception spread code of a received signal. In this case, if the SIR of the received signal falls below a pre-set fixed threshold for a predetermined number of frames continuously, in the mobile communication terminal, it is decided that the reception spread code and the local spread code are in an out-of sync state.
Conventionally, a mobile phone of this type comprises a common channel level measuring unit 1, a received data decoding unit 2, an individual channel level measuring unit 3, and an RLF (Radio Link Failure) decision control unit 4, as shown in FIG. 5. On this mobile phone, a radio signal transmitted from a radio base station is received by a radio unit, not shown, and received data ‘in’ is sent from the RF unit. The level of the common channel (that is, pilot signal, Common Pilot Channel, CPICH) of the received signal ‘in’ is measured by the common channel level measuring unit 1, and the common channel level ‘a’ is output from the common channel level measuring unit 1.
In the received data decoding unit 2, a reception spread code of the received data ‘in’ is de-spread using the local spread code, allocated to the local terminal, and demodulated to produce information signal (baseband signal) ‘c’ of the individual channel. The level of the information signal ‘c’ is measured by the individual channel level measuring unit 3, and the SIR data ‘d’ is output from the individual channel level measuring unit 3. In the RLF decision control unit 4, a first threshold corresponding to an in sync state between the local spread code and the reception spread code, and a second threshold corresponding to an out-of sync state are set. If the common channel level ‘a’ is equal to or higher than a predetermined threshold, the RLF decision control unit 4 compares the SIR data ‘d’ with the first threshold and with the second threshold, for deciding in sync state or out-of sync state between the local spread code and the reception spread code, and outputs the decision signal ‘e’. The decision signal ‘e’ is sent to an individual channel processing unit not shown.
In addition to the mobile phone described above, the conventional, related technologies are described, for example, in the documents given below.
A receiving device and a sending device used in a spread spectrum communication system described in Patent Document 1 comprise a correlation/synchronization decision unit that decides the synchronization using the correlation between a received spreading signal and a spreading code, an RSSI (Receive Signal Strength Indicator) value detection unit that detects the radio wave strength of the received signal, and a BER measuring unit that measures the BER (Bit Error Rate) of the received signal. Based on the acquired result, the quality of the communication channel currently in use is decided by the terminal interface. Because the BER is taken into consideration in deciding the quality of the communication channel, the decision is made if there is an interference wave or a noise from other channels. When the communication channel quality is poor, the communication channel is moved to maintain the communication quality.
In a synchronization decision control device described in Patent Document 2, an SIR measuring unit measures the SIR based on the received signal. A speed detection unit detects the moving speed of a mobile station. A synchronization decision processing unit determines if the detected moving speed is higher than the speed detection threshold. If it is determined that the detected moving speed is higher than the speed detection threshold, the synchronization decision processing unit changes the SIR threshold to a higher value. If it is determined that the detected moving speed is not higher than the speed detection threshold, the synchronization decision processing unit changes the SIR threshold to a lower value. Further, the synchronization decision processing unit decides synchronization based on the SIR that is measured using the changed SIR threshold as a reference.    Patent Document 1:    Japanese Patent Kokai Publication No. JP-P2002-101006A (Abstract, FIG. 1)    Patent Document 2:    Japanese Patent Kokai Publication No. JP-P2005-253055A (Abstract, FIG. 1)    Non-Patent Document 1:    3GPP standard [TS25.331 8.5.6 Radio link failure criteria and actions upon radio link failure]